An international team, including researchers at McGill University, has discovered that low-oxygen aquatic environments around the world contain dissolved manganese (III), which was previously thought to exist only in solid oxides. The finding shows that the metal is more mobile than previously thought and could indicate new classes of bacteria that survive by chemically oxidizing or reducing manganese.
All organisms, including aquatic bacteria, survive by stripping electrons from one substance and transferring them to another. Usually, the electron acceptor is oxygen (O2), but in low-O2 environments, some aquatic bacteria can breathe minerals instead, using compounds like manganese (III) oxide (Mn2O3) and manganese (IV) oxide (MnO2) as electron acceptors. Conversely, there are organisms that survive by reducing these compounds, producing ions of Mn2+ that remain dissolved in the water.
Over the past few years, earth scientists studying low-O2 environments have noticed an anomaly: measurements of dissolved manganese done by atomic absorption spectrometry give different results than those done by electrochemical methods. “Obviously there’s a form of dissolved manganese that isn’t electro-active like Mn2+,” says Alfonso Mucci, a professor of marine geochemistry at McGill University. In a recent paper in Science, Mucci and his team measured dissolved manganese in low-O2 sediments from the St. Lawrence Estuary. In the first few centimeters of the sediment, up to 90 per cent of the dissolved manganese was in the form of Mn3+, which was surprising as this ion is considered unstable in water. Other researchers found the same thing in samples from the Black Sea and salt marshes in Delaware. “Wherever we looked, it was there,” says George Luther, a professor of oceanography at the University of Delaware and one of Mucci’s collaborators.
The teams speculated that dissolved Mn3+ is stabilized by binding with organic ligands produced by bacteria, which then either oxidize it to Mn4+ or reduce it to Mn2+. “It’s something new, a bacterium specialized to use this Mn3+ as an oxidant or a reductant,” says Mucci. The team hopes to return to the estuary next summer, isolate the organisms responsible and identify the reactions that involve Mn3+.